Vascular malformations of the brain affect over 3 million Americans, and cause hemorrhagic stroke and serious neurological disability or death in a significant proportion of affected individuals. Brain arteriovenous malformations (AVMs), one of the two most common types of vascular malformation, are complex vascular lesions, comprised of a tangle of blood vessels that shunts blood from the arterial to the venous circulation without an intervening true capillary bed. Brain AVMs have a population prevalence of 10 to 18 per 100,000 adults, and are an important cause of hemorrhagic stroke in young adults. The localized and often sporadic and solitary nature of the lesion suggests the hypothesis that deleterious somatic mutations underlie brain AVM etiology. For AVMs occurring in the familial syndrome HHT, the two- hit hypothesis has been proposed, where a second, somatic mutation or "hit" in the same gene that is heterozygously mutated in the germline is required for the AVM lesion to form. In sporadic brain AVMs, the first, germline, hit may be de novo;followed by a second, somatic hit, or there may be two somatic hits. AVM lesions are complex and heterogeneous;somatic mutations may only be present in a fraction of lesion cells. Therefore, until recently, detection of somatic mutations in AVMs has been difficult. The recent advent of massively-parallel next generation sequencing technology now suggests a novel, highly sensitive and high- throughput approach to somatic mutation detection in brain AVMs and other complex lesions. To test the hypothesis that somatic mutations underlie brain AVM etiology, both in the inherited (HHT) and the sporadic form of the disease, we will implement massively-parallel next generation sequencing technology to detect somatic mutations in DNA extracted from AVM lesion tissue. We will use this technology to resequence 250 kilobases of DNA from at least 40 candidate genes, including the 3 genes heterozygously mutated in HHT, in AVM lesion tissue from 100 sporadic brain AVM patients and 6 HHT patients. Validation of massively high throughput sequencing for somatic mutation detection in vascular lesions will be an important advance applicable to a variety of vascular and neoplastic lesions. Demonstration of the somatic mutation hypothesis will represent a significant milestone in understanding the mechanisms of brain AVM etiology. In the long term, we will study the relationship between somatic mutations identified and the clinical course of the disease, with a long-term goal of bridging the divide between understanding basic biological mechanisms and risk stratification for AVM clinical management. PUBLIC HEALTH RELEVANCE: Brain arteriovenous malformations (AVMs) are an important cause of hemorrhagic stroke in young adults, resulting in a considerable public health burden encompassing high healthcare costs and lost productivity. The causes of brain AVMs are poorly understood. We will investigate whether brain AVMs, both sporadic and those occurring in the familial disease Hereditary Hemorrhagic Telangiectasia, are caused by somatic mutations in the cells of the AVM lesion. This work will enhance understanding of the molecular mechanisms of AVM formation and hemorrhage, with the long-term goal of improving clinical management and developing new treatment approaches to lessen the negative burden AVMs on health.